Pitting resistivity of Ni-based bulk metallic glasses in chloride solution

Abstract

Resistance of new Ni₇₀Cr₂₁Si_0.5B_0.5P₈C_≤0.1Co_≤1Fe_≤1 (VZ1) and Ni_72.65Cr_7.3Si_6.7B_2.15C_≤0.06Fe_8.2Mo₃ (VZ2) glassy alloys to pitting corrosion was studied in 0.25 M sodium nitrate solution, with or without addition of chloride ions, using electrochemical impedance spectroscopy (EIS), cyclic polarization (CP) and electrochemical frequency modulation (EFM) techniques. Pitting and passivation potentials, corrosion current density and corrosion rate were analyzed relative to the chloride ion concentration. Focusing on the anodic part of the CP scan, the Ni-based glassy alloys in neutral solution acquired passive behavior before the onset of rapid alloy dissolution at the pitting potential. This potential was greatly reduced by the addition of Cl⁻ ions. The aggressive ions led to a sharp increase of current at potentials significantly below the value required for pitting in the neutral medium and enhanced the formation and growth of the pits. A wider potential range free from pitting corrosion was observed for the VZ1 alloy. X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and atomic force microscopy (AFM) were conducted to assess the composition and morphology of the corrosion products and to provide supporting evidence for the electrochemical measurements.